This present invention relates to the practical use of silver sulfide as a rechargeable cathode for batteries and fuel cells utilizing ammonia electrolytes.
The attractiveness of liquid ammonia electrolytes for batteries is largely due to the comparatively low freezing point of ammonia and to the high conductivity of electrolytes in this solvent. Such properties provide for efficient operation of liquid ammonia batteries at subzero temperatures, whereas the performance of batteries utilizing aqueous electrolytes greatly deteriorates at subzero temperatures due to increased viscosity or freezing of the solvent.
The use of cathodes where sulfur species undergo changes in oxidation states by acting as electron-acceptors in the cathode reaction are known in the prior art as shown by U.S. Pat. Nos. 2,689,876; 3,082,284 and 3,121,028. However, the use of sulfur species as electron-acceptors is quite different from the present invention, using a silver sulfide cathode, where monovalent silver atoms act as electron-acceptors. Also, the use of silver sulfide as a consumable cathode in liquid ammonia batteries is in direct contrast with the use of Ag.sub.2 O and Ag.sub.2 S as catalysts for other electrode reactions in aqueous electrolytes as can be found in U.S. Pat. No. 3,386,859. Materials which function as catalysts do not get consumed by the reaction or undergo any net changes in oxidation states. The silver sulfide or silver polysulfide in the instant invention in consumed in the cathodic reaction and has no catalytic function as such. Other metallic sulfide compounds do not have the unique properties in liquid ammonia as do silver sulfides.
Liquid ammonia batteries generally use soluble cathode materials such as m-dinitrobenzene. The performance of these batteries is limited by the rate and extent of the solubility of the cathode material. Also, some of the dissolved cathode material may be lost by undesired side reactions or by chemical reaction with the anode material. Furthermore, the performance of a soluble cathode material can be limited by mass movement to the electrode surface, by the active electrode surface area available and by electrosorption onto the electrode. These limitations do not exist when the insoluble, electronically conducting silver sulfide cathode of this invention is used in liquid ammonia batteries.